Reverse DNS is a system whereby a given IP address is converted into the host name or the domain name. It is a means of validating the IP address. Many web service providers use the reverse DNS system to curtail spam mail. So, if the IP address cannot convert itself into domain name then the mail is blocked. This device has been put to use by a number of service providers. However, some of them discontinued this service, as a lot of their email account holders could not forward the email, even on valid ids.
Reverse DNS also functions as a validation service. Sometimes a domain may lose connection from the Internet due to incompatibility between an IP address and domain name service. This may also cause the website to be inaccessible to others. In such situations the DNS host is required to reallocate an IP address to the domain name. This may take 24 to 72 hours, before the site is back on track.
In order to provide reverse DNS service to a client, first the client must have a domain name registered, and secondly must have an ISP address. The ISP is what makes the DNS server respond to the request of reverse DNS. It is the most important factor to retain an IP address to domain name, or host name.
The opposite of reverse DNS is the standard DNS, which is the means of converting a domain name to an IP address. In that case there has to be a connection between the DNS server and the registrar (domain name registrar).
For the safety and protection of the website, webpage or portal, the reverse DNS should be active. Especially in today's times, where spam mail has become a menace but spam blockers may at times block authentic mails.